1. Field of the Invention
The invention relates to a sewing machine, and in particular to a buttonhole sewing machine.
2. Background Art
U.S. Pat. No. 4,919,061 teaches a sewing machine, i.e. a buttonhole sewing machine for the sewing of linen buttonholes, which comprises a needle drivable to shuttle up and down along a traveling path, a hook with a stock of thread, and a workpiece clamp for a workpiece to be held and guided under the needle during a sewing job. The workpiece clamp comprises a hole for the needle to pass through when a seam is produced. The workpiece clamp is driven by means of a radial cam, i.e. a group of stitches is sewn, in which the beginning and the end of the seam are close together as is the case with buttonholes. The sewing machine further comprises a needle-thread clamp and cutter which is provided with needle-thread-clamping scissors. These consist of a lower half with a cutting edge located at a tip portion. They further consist of an upper half pivotal relative to the lower half about a pivot hinge and likewise provided with a cutting edge at its tip portion. Finally, the needle-thread-clamping scissors comprise a clamping jaw which is pressed against, and springs towards, the tip portion of the upper half for a tail piece of the needle thread to be clamped and held. Regardless of the two positions of the upper half of the scissors relative to the lower half, the needle-thread-clamping scissors, for execution of their working motion in the direction of the lengthwise extension of the buttonhole to be sewn, must be moved into varying positions relative to the stationary needle. This leads to the fact that the tail piece held in the needle-thread-clamping scissors is comparatively long (approximately 10 mm). As a result, reliable sewing over the tail piece by the starting stitches of a buttonhole then to be produced is not achievable so that the quality of the buttonhole is negatively affected. For the performance of the various motions of the needle-thread-clamping scissors, drive mechanisms are required, which are complicated constructionally and in terms of manufacturing engineering.
U.S. Pat. No.4,502,404 teaches another needle thread cutter for a buttonhole sewing machine, in which the problems described above also occur.